The title compound, [Rh2(C2H3O2)4(C21H23NO)2], is a centrosymmetric carboxylate-bridged dirhodium dimer, with each RhII ion in a slightly distorted octahedral coordination environment. One axial bond involves the oxo O atom of the 1-(adamant-1-yliminiomethyl)-2-naphtholate ligand, with a longer than normal Rh-O bond distance which is likely to be due to the bond-lengthening effect of the Rh-Rh bond trans to this O atom. A strong intramolecular hydrogen bond exists between the H atom of the imino N atom and the oxo O atom of the ligand. In addition, the 1-(adamant-1-yliminiomethyl)-2-naphtholate ligand shows delocalization of -electron density over the sequence of five atoms that extends from the N atom to the O atom.

Dirhodium carboxylate bridged species with a variety of axially ligated bases have been widely studied (Cotton et al., 1971; Boyar & Robinson, 1983). Their diamagnetism and short Rh(II)···Rh(II) distance is explained by a Rh—Rh single bond. Weak axial ligation is attributed to the trans-influence of this metal-metal bond. Some of the recent interest in these species has to do with their ability to interact with nucleoside bases. Since a very large number of related compounds has been characterized by X-ray crystallography, a wealth of information about ligand basicities has been made available. The title compound has a Schiff base axial ligand which cannot coordinate in the usual bidendate manner due to the unavailability of cis-coordination sites. Instead, it coordinates through the O donor rather than the N donor atom. The imino nitrogen H atom forms a strong intramolecular H bond to the coordinated O (Fig. 1). This type of coordination has been previously reported in a Zn complex (Zhao, et al., 2003). A bidentate mode of coordination of this ligand has been reported in the bis-Cu(II) complex (Acevedo-Arauz et al., 1992).

The selected geometric parameters show that the Rh—Rh distance in the title compound, 2.3983 (7) Å, is in the normal range for complexes of this type. The axial ligand Rh—O distance is 2.250 (3) Å, considerably longer than the average Rh—O(acetato) distance of 2.043 (3) Å. This is commonly attributed to the trans- bond lengthening effect of the Rh—Rh bond. The bond distances within the axial ligand also show delocalization of the π electron density over the five atoms, N1, C11, C10, C1 and O5. Resonance structures that contribute to this delocalization give rise to zwitterions that place positive charge on N1 and negative charge on C10 or O5, thus enhancing the hydrogen bond and causing C9—C10 to be long and C2—C3 to be short. The entire 2-napthol-iminomethyl group is planar with an average deviation from the plane of 0.032 (5) Å. This pattern of protonation and bond distances persists in the free ligand as well (Acevedo-Arauz et al., 1992).

For the structures of the ligand and a copper complex of the ligand, see: Acevedo-Arauz et al. (1992); for a related zinc complex, see: Zhao et al. (2003); for a review of rhodium(II) carboxylates, see: Boyar & Robinson (1983); for a primary structure of the aqua carboxylate compound, see: Cotton et al. (1971).

The ligand, 1-(1-adamantyl)iminomethyl)-2-napthol, was prepared in the follwing manner. 1-Aminoadamantane (100 mg) and 2-hydroxy-1-naphthaldehyde (114 mg) were placed in a round bottom flask. Then 40 ml of ethanol was added to the mixture, and the solution was stirred for 3 hrs. The solution was then cooled to room temperature and the volume of solution was reduced by half. After the solution was placed in an ice bath, a yellow precipitate formed, which was filtered and collected. To prepare the title compound, in a 100 ml round bottom flask, 46 mg of 1-(1-adamantyl)iminomethyl)-2-napthol was dissolved in 30 ml of methanol, followed by addition of 16 mg of [Rh(OAc)2]2.2(H2O)2 dissolved in 15 ml of methanol. The reaction mixture was stirred for 3 hrs at room temperature. The solution was then filtered to remove any solids and the solvent removed by use of a rotary evaporator. The product material was dissolved in diethyl ether and allowed to slowly evaporate. This method afforded green crystals, yield, 47%.

The C-bound H atoms were positioned geometrically with C—H = 0.95–1.00 Å, and allowed to ride on their parent atoms with Uiso(H) = 1.2 Ueq(C), 1.5Ueq(C) for methyl groups. The hydrogen atom bonded to N1 was freely refined. The structure is a rotational twin which was separated by use of cell_now and TWINABS (Bruker, 2007) into two domains related by a 180° rotation about the real [0 − 1 1] axis. The twin parameter refined to 0.3968 (13) using all observations involving domain 1 (HKLF 5).

Fig. 1. A view of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity except for the imino H atom that is involved in hydrogen bonding, which is shown as a sphere of arbritrary size. Symmetry code: (A) −x,-y, 1 − z.

Tetra-µ2-acetato-κ8O:O'-bis{[1-(1-adamantyliminomethyl)- 2-naphthol-κO]rhodium(II)} top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.